From the second edition (1989):

pur-, prefix

The usual AF. form of OF. por-, pur-, mod.F. pour-:—L. pōr-, prō-, prep. and prefix (see pro- prefix1). The form in which this prefix came into early ME. through OF., still retained in numerous words as purchase, purfle, purlieu, purloin, purport, purpose, purpresture, pursue, purvey, and their derivatives, as well as in the earlier forms of some words in which it has been since altered to the L. form, as promenade, etc. See the individual words.